Shags consume what comes along rather than foraging optimally

Abstract

Even though food and energy are the ultimate rewards when an animal goes foraging, the effort and time they invest in pursuit of a meal directly detract from the profits of their exertions. Consequently, animals invest in tried and tested foraging habits to remain ahead of the energy game. ‘Foraging theory predicts that an individual is more likely to remain at a feeding site if its feeding rate is high, but move to another site in the same or a different habitat if the feeding rate is low’, says Yutaka Watanuki from Hokkaido University, Japan. Essentially, most animals try to optimise the effort they invest when foraging, based on past experience. However, it was almost impossible to determine how much an animal consumes to assess whether they really do forage optimally. That was until Waktanuki and colleagues came up with a clever way of determining how much a seabird consumes by comparing the rate at which it flaps its wings before and after feeding. Having solved that problem, Waktanuki, Katsufumi Sato and Kozue Shiomi (The University of Tokyo, Japan), with Sarah Wanless and Francis Daunt (UK Centre for Ecology & Hydrology), attached motion sensors to 28 male European shags (Gulosus artistotelis) during the breeding season, when the birds head out to sea to forage for their young, to discover how much the birds consume in sandy and rocky dining sites and to determine whether shags conform to optimal foraging rules or use a more opportunistic approach.However, when the team assessed how much the marine birds were consuming as they feasted on shoals of sandeels in sandy areas, or more dispersed fish in rocky locations, the shags essentially consumed the same amount, even though they should have eaten more sandeels if foraging optimally, because more are available. And when the team checked whether the birds switched feeding location swiftly when dining opportunities proved unsatisfactory, the birds did not, ‘in contrast to the predictions of optimal foraging theory’, says Daunt. Instead, the birds appear to feed opportunistically, consuming whatever comes their way, rather than basing their dining preferences on previous experience to optimise their foraging.‘Shags feed communally with many other shags and other seabirds, particularly when they are feeding on sandeels’, says Wanless, explaining that the optimal foraging rules that work so well for many songbirds that have discrete feeding territories might break down when animals have to take their chances and dine alongside neighbours from their breeding colony.